Project Summary The Centers for Disease Control and Prevention has estimated that there are approximately 6.1 million infertile couples with a female spouse aged 15-44 in the U.S., which is about 6.7% of the domestic married couple population base for that age group. Miscarriage before 20 weeks also occurs in about 15% of known pregnancies, and over 75% of failed pregnancies involve implantation defects. To solve these problems, we must understand the mechanisms of uterine receptivity and implantation to develop better treatments that may be currently out of reach. The endometrium's epithelial and stromal compartments undergo dynamic molecular and morphological changes to prepare for implantation and development. Endometrial P4 resistance implies a decreased responsiveness of target tissue to bioavailable P4, and such an impaired P4 response is seen in the endometrium of women with non-receptive endometrium. However, exactly how P4 signaling becomes defective in a non-receptive endometrium is still unclear. MIG-6 acts as a key P4 signaling mediator to inhibit E2-mediated epithelial proliferation in the endometrium of the human and mouse. We hypothesize that Mig-6 loss causes endometrial P4 resistance by ErbB2 overexpression in the endometrium and by dysregulating P4 signaling in endometrial stromal cells. In this proposal, our objective is to determine how MIG-6 functions in the uterus and how it is dysregulated in endometrial P4 resistance and infertility. Our Specific Aims are directed at understanding: 1) the pathophysiological role of MIG-6 loss in implantation failure; and 2) the effect of Erbb2 ablation on female infertility with Mig-6 deficiency. Using our mouse models and biomedical imaging techniques, we will determine the role of Mig-6 loss in implantation failure and test Erbb2 targeting to treat endometrial P4 resistance and restore implantation. Our results will enhance our understanding of reproductive pathophysiology as well as enable the development of more effective strategies for the diagnosis and treatment of infertility.